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Calc Easy Help

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SushiMan
Posts: 1269 - Karma: 424
FInals tomorrow and I dont understand how to find the X values at which f(x) = (-x^2+1)(x^-3) has a horizontal tangent. I got as far as 0=x^-2-3x^-4 using the product rule but I cant get any farther... please need help!
Jan 21 2010 4:18AM
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moonofendor
Posts: 1484 - Karma: 14
f(x) = (-x^2+1)(x^-3)

so (-2x)(x^-3)+(-x^2+1)(-3x^-4)

then set all that equal to zero. so it has horizontal tangents when x=0.
and when (-2x)(x^-3)=-(-x^2+1)(-3x^4)
Jan 21 2010 4:33AM
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SushiMan
Posts: 1269 - Karma: 424
thanks what is the answeer you got'? and i dont understand the bottom

Jan 21 2010 4:40AM
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moonofendor
Posts: 1484 - Karma: 14
there's going to be a point when (-2x)(x^-3)=-(-x^2+1)(-3x^4). whenever this happens the tangent line will also equal zero.


Jan 21 2010 5:08AM
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i.ski.walls
Posts: 94 - Karma: 25
all you do is find the derivative of the function, and find the zeros for the derivative. this should give you the horizontal tangets of the orginal graph.
Jan 21 2010 1:12PM
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sstricky
Posts: 1876 - Karma: 111
this but wrong...

so you find the deriv which is  (-2x)(x^-3)+(-x^2+1)(-3x^-4)

then just set that equal to 0.... 0 = (-2x)(x^-3)+(-x^2+1)(-3x^-4)

then solve for x.. that is your answer
Jan 21 2010 1:15PM
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Waffle11
Posts: 481 - Karma: 16
Jan 21 2010 1:16PM
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chapz
Posts: 10182 - Karma: 66
pretty sure this is right.

Jan 21 2010 1:23PM
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I_Am_Mod
Posts: 13214 - Karma: 3,753
http://en.wikipedia.org/wiki/L'Hôpital's_rule
Jan 21 2010 1:25PM
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ClownPrince
Posts: 500 - Karma: 10
correct me if i'm wrong, but i don't think that has anything to do with the problem. interesting, though.
Jan 21 2010 1:54PM
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SushiMan
Posts: 1269 - Karma: 424
so i just took the finally and most likely failed it fuck my life haha i studied 12_plus hours this week
the class is not even an AP, its only college prep and now i think im failing it! great

any tips on math? i cant seem to connect the dots to figure out problems...
the way i have mistakenly learned math is just memorizing specific problems and how to do those. anyone else have the same problem?
Jan 21 2010 2:03PM
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i.ski.walls
Posts: 94 - Karma: 25
yeah i guess im in calc BC and i'm taking a final on AB calc right now so i guess i could help. if you were to give me the topics that your covering right now i could help you out.

calcs a bitch. i'm not gonna lie.
Jan 21 2010 2:22PM
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Chickendodo
Posts: 1426 - Karma: 33
You might hate me then, I'm a math major =], I think math is fun!  As to whoever said they're taking AB, AB is pie, I hear BC is a bitch though.
Jan 21 2010 2:29PM
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